The invention relates to light emitting diodes having a heat management system, and more particularly to a light emitting diode having a thermally-conductive structure for dissipating heat.
Light emitting diodes (LED) have been available since the early 1960""s. Because of the relatively high efficiency of LEDs, LED usage has greatly increased in popularity in all types of applications. Recent developments in making high temperature and high brightness LEDs have expanded the use of LEDs from signs and message boards to automobile interior and exterior lights and even traffic signals. Even with new high-temperature LED technology, however, LEDs still exhibit a substantial decrease in light output when the temperature of the LED junction increases. For example, an increase of 75xc2x0 C. at the junction temperature may cause the level of luminous flux to be reduced to one-half of its room temperature value. This phenomenon limits the amount of output from conventional LEDs.
LEDs are often encapsulated in an optically clear epoxy resin, which is a poor thermal conductor. Because of the epoxy""s poor thermal conductivity, very little heat can dissipate from the LED through the optical epoxy. This restriction places a severe limit on the drive current that can be used to drive the LEDs because any excess current would exceed the temperature limitations of the LED and result in a decrease in light output.
In addition, commonly used epoxy resins have a glass transition temperature at which the resin transforms from a rigid, glass-like solid to a rubbery material. A dramatic change in the coefficient of thermal expansion of the LED is generally associated with the glass transition temperature. This may cause a mechanical failure in the LED (e.g., components of the LED may separate) or cause the bounding wire in the LED to break.
For LEDs having low thermal resistance, the relative flux increases almost proportionally to the forward current. However, for LEDs having high thermal resistance, which describes most LEDs in use today, relative flux can actually decrease as forward current is increased. For LEDs with high thermal resistance, a great deal of heat accumulates in the LED, resulting in high LED junction temperatures. In these cases, the effects of increasing junction temperature can offset the effects of increased forward current, causing the LED to maintain or even lower its light output level even with increases in the forward current due to the LED""s rising junction temperature.
There have been some attempts to create an LED structure that has more efficient heat dissipation so that higher forward currents will increase, rather than decrease, the LED""s light output. U.S. Pat. No. 5,857,767 to Hochstein discloses a way to mount LEDs to a heat sink with an electrically conductive epoxy. This structure does allow LEDs to be driven with more current than conventional structures while maintaining low junction temperatures, thereby allowing increased light output. However, there are not many LEDs that are compatible with the Hochstein structure because most the LEDs use a lead frame to support the LED chip as well as to make electrical connections. The structure of the lead frame requires any heat in the LED to conduct through long, narrow legs, making it difficult to remove any significant amount of heat from the LEDs junction.
There is a need for an LED structure that can dissipate heat quickly and efficiently so that the junction temperature of the LED can remain at a stable level even when the drive current of the LED is increased to increase light output.
Accordingly, an LED structure according to the invention constructs an anode portion and a cathode portion from thermally conductive material. An LED chip is supported by the cathode portion, allowing any generated heat from the LED chip to be carried away via the thermally conductive material. The inventive structure is preferably designed to couple with a heat sink for efficient thermal dissipation.
The inventive structure creates an LED having a larger cross-sectional area and a relatively short path between the LED chip and the heat sink, increasing the efficiency in which heat is directed away from the LED. The efficient heat removal properties of the inventive LED structure allows the LED junction temperature to be kept low even as the forward current through the LED chip is increased to increase light output from the single LED. As a result, the inventive LED structure allows the LED to be driven with a much larger current, allowing increased overall light output or allowing use of fewer LEDs to provide a given output.